Solar cell

ABSTRACT

A method for fabricating a solar cell, includes forming an emitter layer by doping a first impurity having a second conductivity type, opposite a first conductivity type, on a front surface of a substrate having the first conductivity type; forming a back surface field by doping a second impurity having the first conductivity type on a rear surface of the substrate; and forming a plurality of front finger lines in contact with the emitter layer and a plurality of rear finger lines in contact with the back surface field, wherein the emitter layer has a selective emitter structure, the back surface field has a selective back surface field structure, and the number of the plurality of rear finger lines positioned on the rear surface of the substrate is different from the number of the plurality of front finger lines positioned on a front surface of the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. application Ser. No.13/426,908, filed on Mar. 22, 2012, which claims priority to KoreanPatent Application No. 10-2011-0056900, filed on Jun. 13, 2011 in theKorean Intellectual Property Office, all of which are herebyincorporated herein by reference into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

Embodiments of the invention relate to a solar cell and, moreparticularly, to a solar cell in which a structure of an emitter layeris the same as that of a back surface field.

2. Description of the Related Art

Recently, as existing energy resources such as oil or coal are expectedto be exhausted, an interest in alternative energy sources for replacingthe oil or coal is increasing. In particular, a solar cell whichdirectly converts (or transforms) solar energy into electric energy byusing a semiconductor element is getting the spotlight as anext-generation cell.

Namely, a solar cell is a device for converting (or transforming) lightenergy into electric energy by using photovoltaic effects, and may beclassified into a silicon solar cell, a thin film type solar cell, adye-sensitized solar cell, an organic polymer type solar cell, and thelike, according to constituent parts. In a solar cell, it is importantto increase conversion efficiency (or transformation efficiency) inrelation to a rate at which incident solar light is converted (ortransformed) into electric energy.

SUMMARY OF THE INVENTION

An aspect of the invention provides a solar cell having improvedphotoelectric conversion efficiency.

According to an aspect of the invention, there is provided a solar cellincluding: a substrate having a first conductivity type; an emitterlayer including a first impurity having a second conductivity typeopposite the first conductivity type, doped therein, and positioned onone surface of the substrate; a plurality of front finger linesconnected with the emitter layer; a back surface field positioned on theother surface of the substrate facing the one surface of the substrate,and including a second impurity having the first conductivity type dopedtherein; and a plurality of rear finger lines connected with the backsurface field, wherein the emitter layer includes first areas in contactwith the plurality of front finger lines and second areas positionedbetween the plurality of front finger lines and having a lower dopingconcentration than that of the first areas, wherein the back surfacefield includes areas in contact with the plurality of rear finger lines,and wherein the number of the plurality of rear finger lines positionedon the other surface of the substrate and the number of the plurality offront finger lines positioned on the one surface of the substrate aredifferent.

According to another aspect of the invention, there is provided a methodfor fabricating a solar cell, including: forming an emitter layer bydoping a first impurity having a second conductivity type opposite afirst conductivity type, on a front surface of a substrate having thefirst conductivity type; forming a back surface field by doping a secondimpurity having the first conductivity type on a rear surface of thesubstrate; and forming a plurality of front finger lines in contact withthe emitter layer and a plurality of rear finger lines in contact withthe back surface field, wherein the emitter layer has a selectiveemitter structure, the back surface field has a selective back surfacefield structure, and the number of the plurality of rear finger linespositioned on the rear surface of the substrate is different from thenumber of the plurality of front finger lines positioned on a frontsurface of the substrate.

According to embodiments of the invention, since the emitter layer andthe back surface field have the same structure, the photoelectricconversion efficiency of the solar cell can be improved.

The foregoing and other objects, features, aspects and advantages of theinvention will become more apparent from the following detaileddescription of the invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a solar cell according to an embodimentof the invention.

FIG. 2 is a sectional view taken along line A-A′ of the solar cell ofFIG. 1.

FIG. 3 is a sectional view taken along line B-B′ of the solar cell ofFIG. 1.

FIGS. 4 to 9 are views showing a method for fabricating the solar cellof FIG. 1.

FIG. 10 is a sectional view of a solar cell module according to anembodiment of the invention.

FIG. 11 is an enlarged view of a portion ‘C’ of the solar cell module ofFIG. 10.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Example embodiments of the invention will now be described in detailwith reference to the accompanying drawings.

It will be understood that when an element or layer is referred to asbeing “on” or “connected to” another element or layer, it may bedirectly on or directly connected to the other element or layer, orintervening elements or layers may be present, and reference for anupper or lower element or layer will be described based on the referenceof the drawings. In the drawings, the shapes and dimensions may beexaggerated, omitted, or schematically illustrated for clarity.

FIG. 1 is a perspective view of a solar cell according to an embodimentof the invention. FIG. 2 is a sectional view taken along line A-A′ ofthe solar cell of FIG. 1. FIG. 3 is a sectional view taken along lineB-B′ of the solar cell of FIG. 1. In this instance, FIGS. 2 and 3 aresectional views in a Z direction after cutting the solar cell of FIG. 1into an X-Y plane.

With reference to FIGS. 1 to 3, a solar cell 100 according to anembodiment of the invention may include a first conductive siliconsemiconductor substrate 110, an emitter layer 120 positioned on onesurface of the substrate 110, a plurality of front finger lines 140connected with the emitter layer 120, a back surface field 150 formed onthe other surface of the substrate 110, and a plurality of rear fingerlines 170 connected with the back surface field 150. Also, the solarcell 100 may include a first anti-reflective film 130 on the emitterlayer 120 and a second anti-reflective film 160 on the back surfacefield 150.

First, the substrate 110 may be made of silicon and it may have a firstconductivity type based on a P type or N type impurity being dopedtherein. For example, a group III element, such as boron (B), gallium(Ga), indium (In), or the like, may be doped as an impurity in siliconto implement a P type substrate, and a group V element, such asphosphorus (P), arsenic (As), antimony (Sb), or the like, may be dopedin silicon to implement an N type substrate.

Meanwhile, the surface of the substrate 110 may have an irregularstructure (or a depression and protrusion structure). The irregularstructure refers to texturing of the surface of the substrate 110 toform irregular patterns. When the substrate 110 is textured thusly, theemitter layer 120, the first anti-reflective film 130, the back surfacefield 150, and the second anti-reflective film 160 may be formedaccording to an irregular shape of the irregular patterns. Thus, thereflectance (or reflectivity) of light made incident to the solar cell100 can be reduced and the amount of optical traps can be increased,thereby reducing an optical loss of the solar cell 100.

The emitter layer 120 is formed by doping into the substrate 110 a firstimpurity having a second conductivity type which is the opposite to thatof the substrate 110. For example, when the substrate 110 is a P typesubstrate, an N type impurity is doped or included in the emitter layer120, and when the substrate 110 is an N type substrate, a P typeimpurity is doped or included in the emitter layer 120. In this manner,when impurities having the mutually opposite conductivity types aredoped in the substrate 110 and the emitter layer 120, respectively, aP-N junction is formed on the interface between the substrate 110 andthe emitter layer 120.

Meanwhile, the emitter layer 120 may include first areas 124 in contactwith the plurality of front finger lines 140 and second areas 122disposed between the plurality of front finger lines 140, namely,between the plurality of first areas 124. The second areas 122 are notin contact with the plurality of front finger lines 140.

In general, as the impurity of the emitter layer 120 increases, a largernumber of electron-hole pairs generated by a photoelectric effect can berecombined. Thus, it is advantageous for the second areas 122, in whichlight is largely converted into electron-hole pairs, to have arelatively low impurity concentration, while it is advantageous for thefirst areas 124, in which separated electrons or holes move to the frontfinger lines 140, to have a relatively high impurity concentration inorder to reduce contact resistance.

Thus, in the solar cell 100 according to an embodiment of the invention,in order to reduce contact resistance with the front finger lines 140and reduce or prevent a degradation of efficiency of the solar cell 100according to the surface recombination, the solar cell 100 may have aselective emitter structure in which the first areas 124 where the frontfinger lines 140 are positioned are formed as a portion of the emitterlayer 120 having a relatively high concentration. Accordingly, surfaceresistance of the first areas 124 of the emitter layer 120 is reduced tobe smaller than that of the second area 122.

The first areas 124 of the emitter layer 120 may have a dopingconcentration ranging from 1E19 to 1E21, and the second areas 122 of theemitter layer 120 may have a doping concentration ranging from 5E18 to1E20. In this instance, surface resistance of the first areas 124 of theemitter layer 120 may be set to range from 30 Ω/sq. to 70 Ω/sq.,preferably, from 40 Ω/sq. to 70 Ω/sq., and that of the second areas 122may be set to range from 70 Ω/sq. to 150Q/sq., preferably, from 90 Ω/sq.to 120 Ω/sq.

Meanwhile, the thickness (b) of the first areas 124 may be greater thanthe thickness (a) of the second areas 122. First, the first areas 124may be formed to have a thickness ranging from 0.5 μm to 2 μm. If thethickness (b) of the first areas 124 is smaller than 0.5 μm, a shunt mayoccur with the emitter layer 120 due to a fire through operationperformed in forming the front finger lines 140, while if the thickness(b) of the first areas 124 is greater than 2 μm, sites of recombinationof minority carriers increase to result in a greater reduction of shortcircuit current density (Jsc).

Also, the second areas 122 may be formed to have a thickness rangingfrom 0.3 μm to 1 μm. If the thickness (a) of the second areas 122 issmaller than 0.3 μm, a fill factor may be reduced due to an increase inresistance, and if the thickness (a) of the second areas 122 is greaterthan 1 μm, Jsc may be reduced according to an increase in therecombination.

In particular, since the front finger lines 140 are not positioned onthe second areas 122, a shallow emitter can be formed. Thus, atransmission of blue light having a short wavelength may increase,improving the efficiency of the solar cell 100.

For example, the first anti-reflective film 130 may be formed as asingle film made of one selected from the group consisting of siliconnitride, silicon oxide, silicon oxynitride, intrinsic amorphous silicon,MgF₂, ZnS, TiO₂, and CeO₂, or may have a multi-layered structure inwhich two or more films selected from the foregoing group are combined.When the substrate 110 is a P type substrate, the first anti-reflectivefilm 130 may also serve as a passivation layer.

Thus, the first anti-reflective film 130 removes a recombination site ofcarriers existing on the surface of the emitter layer 120 or in the bulkof the emitter layer 120, and reduces reflectance of solar light madeincident to a front surface of the substrate 110. Accordingly, when therecombination site of the carriers existing in the emitter layer 120 isremoved, an open circuit voltage Voc of the solar cell 100 is increased.Also, when the reflectance of the solar cell is reduced, the quantity oflight reaching the P-N junction is increased to increase a short circuitcurrent Isc. When the open circuit voltage and the short circuit currentof the solar cell 100 are increased, the conversion efficiency of thesolar cell 100 can be improved as much.

The first anti-reflective film 130 may be formed to have a refractiveindex ranging from 1.8 to 2.5 and thickness ranging from 60 μm to 100μm. If the refractive index of the first anti-reflective film 130 issmaller than 1.8, an anti-reflection effect of the first anti-reflectivefilm 130 would not be sufficient or good, while if the refractive indexof the first anti-reflective film 130 is greater than 2.5, an opticalabsorption occurs in the first anti-reflective film 130 in a wavelengtharea contributing to a current conversion of incident light and aconversion efficiency is rather degraded.

Meanwhile, when the substrate 110 is an N type substrate, a passivationlayer may be formed between the emitter layer 120 and the firstanti-reflective film 130. The passivation layer may be made of, forexample, SiOx, AlxOy, or the like.

The front finger line 140 may collect electrons or holes generated bythe photoelectric effect. A plurality of front finger lines 140 may beformed.

Meanwhile, when the emitter layer 120 is a P type emitter layer, pasteincluding AgAl, glass frit, and the like, may be screen-printed at aportion, where the front finger lines 140 are to be formed, by using amask with an opening and then thermally treated to form the front fingerlines 140 such that they are in ohmic-contact with the emitter layer120. When the emitter layer 120 is an N type emitter layer, pasteincluding Ag, glass frit, and the like, may be screen-printed and thenthermally treated to form the front finger lines 140.

Also, the first anti-reflective film 130 and/or the passivation layeris/are irradiated with a laser beam through laser ablation so as to beremoved to expose areas of the emitter layer 120, and a seed layer isdeposited with nickel (Ni), or the like, on the removed areas, and then,the front finger lines 140 may be deposited through plating, sputtering,or the like, so as to be formed on the removed areas. The front fingerlines 140 formed thusly may have a structure including nickel/copper/tin(Ni/Cu/Sn), nickel/silver (Ni/Ag), nickel/copper/silver (Ni/Cu/Ag), orthe like, but the embodiments of the invention are not meant to belimited thereto. Also, the front finger lines 140 formed thusly may havea width of 10 μm or greater and a height (or a thickness) ranging from60 μm to 80 μm, but the embodiments of the invention are not meant to belimited thereto. For a comparison, when not using plating, the width ofthe front finger lines 140 may be 30 μm to 60 μm, in cases such asdouble printing, light induced plating, and the like. However,embodiments of the invention may use fine lines of the finger lines 140whereby thicknesses are less than 10 μm and doubly printed.

Meanwhile, the plurality of front finger lines 140 are in contact withfront bus electrodes 180 crossing the front finger lines 140. The frontbus electrodes 180 may be connected with a ribbon to supply currentgenerated in the solar cell 100 to the exterior.

The back surface field 150, which is a highly concentrated doped area,can reduce or prevent the recombination of electron-hole pairs separatedfrom a rear surface of the substrate 110, reduce a leakage current, andallow for obtaining improved ohmic contact characteristics. Such a backsurface field 150 may be formed by doping a second impurity having thesame first conductivity type as that of the substrate 110.

The back surface field 150 may have the same structure as that of theforegoing emitter layer 120. Namely, the back surface field 150 mayinclude third areas 154 in contact with the plurality of rear fingerlines 170 and fourth areas 152 disposed between the plurality of rearfinger lines 170, namely, between the plurality of third areas 154. Adoping concentration of an impurity of the third areas 154 may begreater than that of the impurity of the fourth areas 152. Thus, thesolar cell 100 according to an embodiment of the invention may be abifacial solar cell having the structure of the selective emitter layer120 and the selective back surface field 150.

Meanwhile, the third areas 154 of the back surface field 150 may have adoping concentration higher than that of the fourth areas 152 in orderto reduce contact resistance with the rear finger lines 170. Also, sincethe fourth areas 152 are formed over the entirety of the rear surface ofthe substrate 110, electron hole recombination can be effectivelyreduced or prevented.

Thus, a loss otherwise caused by electron hole recombination can bereduced, and at the same time, since the capability of transferringelectrons or holes, generated in the back surface field 150 according toa photoelectric effect, to the rear finger lines 170 is furtherimproved, the photoelectric conversion efficiency of the solar cell 100can be further improved.

The third areas 154 of the back surface field 150 may have a dopingconcentration ranging from 1E19 to 1E21, and the fourth areas 122 mayhave a doping concentration ranging from 5E18 to 1E20. Also, a surfaceresistance of the third areas 154 may be set to range from 20 Ω/sq. to70 Ω/sq., preferably, from 40 to 60 Ω/sq., and that of the fourth areas152 may be set to range from 60 Ω/sq. to 150 Ω/sq., preferably, from 90Ω/sq. to 120 Ω/sq.

Meanwhile, the third areas 154 may be doped to be thicker than thefourth areas 152. In order to prevent an increase in the shunt andrecombination with the rear finger lines 170, the thickness (d) of thethird areas 154 may range from 0.5 μm to 2 μm, and in order to reduce orprevent an increase recombination and resistance, the thickness (c) ofthe fourth areas 152 may range from 0.3 μm to 1 μm.

In particular, since the rear finger lines 170 are not positioned on thefourth areas 152, the back surface field 150 can be formed to be thin.Thus, a transmission of blue light of having a short wavelength mayincrease, improving the efficiency of the solar cell 100.

Meanwhile, FIG. 3 is a sectional view taken along line B-B′ of the solarcell of FIG. 1. With reference to FIG. 3, the back surface field 150 isformed to be separated by a distance or a gap ‘T’ from the edge of therear surface of the substrate 110. Thereby, an end of the back surfacefield 150 or a fourth area 152 is separated from an end of the emitterlayer 120 or a second area 122 by an intervening portion of thesubstrate 110. This is to prevent a short circuit between the backsurface field 150 and the emitter layer 120, since the emitter layer 120may be formed up to the side of the substrate 110 in doping an impurityto form the emitter layer 120. Thus, an edge isolation process forpreventing a short circuit between the front and rear surfaces of thesubstrate 110 (to be described) can be omitted.

Here, a second impurity may be included in the distance or gap T due todiffusion, or the like, between the back surface field 150 and the edgeof the rear surface of the substrate 110. However, an amount of thediffusion can be negligible because it is an amount that does not causea short circuit between the back surface field 150 and the emitter layer120.

Meanwhile, the distance T between the back surface field 150 and theedge of the rear surface of the substrate 110 may range from 2 μm to 300μm. If the distance T between the back surface field 150 and the edge ofthe rear surface of the substrate 110 is shorter than 2 μm, a shortcircuit would possibly occur between the back surface field 150 and theemitter layer 120 due to impurity diffusion, or the like, which maycause the efficiency of the solar cell 100 to be reduced. Meanwhile, ifthe distance T between the back surface field 150 and the edge of therear surface of the substrate 110 is greater than 300 μm, the area ofthe rear electric field 150 would be reduced to increase recombinationof electron and hole pairs, to thus, reduce the efficiency of the solarcell 100.

The second anti-reflective film 160 may be formed on the back surfacefield 150. The second anti-reflective film 160 may be the same as theforegoing first anti-reflective film 130. Namely, the secondanti-reflective film 160 may be formed as a single film made of oneselected from the group consisting of silicon nitride, silicon oxide,silicon oxynitride, intrinsic amorphous silicon, MgF₂, ZnS, TiO₂, andCeO₂, or may have a multi-layered structure in which two or more filmsselected from the foregoing group are combined, reducing reflectance ofsolar light made incident to the rear surface of the substrate 110.Further, when the substrate 110 is an N type substrate, the secondanti-reflective film 160 may also serve as a passivation layer. Thesecond anti-reflective film 160 may be formed to have a refractive indexranging from 1.8 to 2.5 and thickness ranging from 60 μm to 100 μm.

Meanwhile, when the substrate 110 is a P type substrate, a passivationlayer may be formed between the back surface field 150 and the secondanti-reflective film 160. The passivation layer may be made of, forexample, SiOx, AlxOy, or the like.

A plurality of rear finger lines 170 may be formed. When the substrate110 is an N type substrate, the rear finger lines 170 may be formed byscreen-printing a paste including AgAl, glass frit, and the like, suchthat they are in ohmic-contact with the back surface field 150. When thesubstrate is a P type substrate, a paste including Ag, glass frit, andthe like, may be used to form the rear finger lines 170.

Also, the second anti-reflective film 160 and/or the passivation layeris/are irradiated with a laser beam through laser ablation so as to beremoved to expose areas of the back surface field 150, and a seed layeris deposited with nickel (Ni), or the like on the removed areas, andthen, the second finger lines 170 may be deposited through plating,sputtering, or the like, so as to be formed on the removed areas. Thesecond finger lines 170 formed thusly may have a structure includingnickel/copper/tin (Ni/Cu/Sn), nickel/silver (Ni/Ag),nickel/copper/silver (Ni/Cu/Ag), or the like, but embodiments of theinvention are not limited thereto. Also, the second finger lines 170formed thusly may have a width of 10 μm or greater and a height rangingfrom 60 μm to 80 μm. When not using plating, the width of the frontfinger lines 140 may be 30 μm to 60 μm, in cases such as doubleprinting, light induced plating, and the like. However, embodiments ofthe invention may use fine lines of the finger lines 140 wherebythicknesses are less than 10 μm and doubly printed. That is, theembodiments of the invention are not meat to be limited the above widthand/or height.

The plurality of rear finger lines 170 are in contact with rear buselectrodes 190 crossing the rear finger lines 170 to supply currentgenerated according to a photoelectric effect to the exterior.

Meanwhile, the number of the rear finger lines 170 and that of the frontfinger lines 140 may be different when one of them is increased. Forexample, the number of the rear finger lines 170 may be greater thanthat of the front finger lines 140.

When the number of the rear finger lines 170 is greater than that of thefront finger lines 140, since a movement distance of electrons or holesto the rear finger lines 170 is shortened, overall resistance of thesolar cell 100 can be reduced. Also, since there is no need to increasethe number of the front finger lines 140 to reduce resistance, a lightabsorption to the front surface of the solar cell 100 is notadditionally interfered with, thus reducing or preventing a reduction ina light absorption rate of the solar cell 100.

Also, a ratio (referred to as ‘D2/D1’, hereinafter) of a distance D1between two adjacent rear finger lines 170 among the plurality of rearfinger lines 170 to a distance D2 between two adjacent front fingerlines 140 among the plurality of front finger lines 140 may be greaterthan 1 and smaller than 5.

If the distance D1 between the rear finger lines 170 is reduced to makethe ratio of D2/D1 be 5 or greater, the overall area of the rear fingerlines 170 would be increased to reduce an open circuit voltage Voc ofthe solar cell 100 according to an increase in recombination of electronand hole pairs due to a metal. Also, if the distance D2 between thefront finger lines 140 is increased to make the ratio of D2/D1 be 5 orgreater, a movement path of carriers up to the front finger lines 170 isincreased to reduce the efficiency of the solar cell 100.

Meanwhile, if the distance D1 between the rear finger lines 170 isincreased to make the ratio of D2/D1 be 1 or smaller, resistanceaccording to an increase in the movement path of carriers would beincreased to reduce the efficiency of the solar cell 100, and if thedistance D2 between the front finger lines 140 is reduced to make theratio of D2/D1 be 1 or smaller, the increased area of the front fingerlines 140 would additionally interfere with solar light made incident tothe rear surface to reduce Jsc. Thus, the ratio of D2/D1 may be greaterthan 1 and smaller than 5.

Also, the distance D1 between two adjacent rear finger lines 170 amongthe plurality of rear finger lines 170 may range from 0.5 μm to 2.5 μm.In this instance, the front finger lines 140 may be formed to satisfythe foregoing value of D2/D1.

If the distance D1 between rear finger lines 170 is smaller than 0.5 μm,the recombination of electron and hole pairs according to an increase inthe overall area of the rear finger lines 170, and the increased area ofthe rear finger lines 170 would interfere with solar light made incidentto the rear surface to reduce Jsc. Meanwhile, if the distance D1 betweenthe rear finger lines 170 is greater than 2.5 μm, the fill factor wouldbe reduced due to an increase in the resistance according to an increasein the movement path of carriers. Thus, preferably, the distance D1between two adjacent rear finger lines 170 ranges from 0.5 μm to 2.5 μm.

FIGS. 4 to 9 are views showing processes of a method for fabricating thesolar cell of FIG. 1.

A method for fabricating the solar cell 100 according to an embodimentof the invention will now be described with reference to FIGS. 4 to 9.(a) of FIG. 4 shows a plan view of the solar cell 100 and (b) of FIG. 4shows a side view of the solar cell 100. First, a first impurity havinga second conductivity type opposite to a first conductivity type isprimarily doped with a first doping concentration on the siliconsemiconductor substrate 110 having the first conductivity type to formthe second areas 122 of the emitter layer 120 as described above.

The substrate 110 may have an irregular structure. The irregularstructure can reduce reflectance of solar light made incident to thesolar cell 100, increasing the amount of optical traps and thus reducingan optical loss of the solar cell 100. The irregular structure may beformed through a process of putting the substrate 110 in etchant, or thelike. The irregular structure may be formed to have various shapes suchas a pyramid, a square, a triangle, or the like.

Meanwhile, the doping of the first impurity to form the second areas 122may be performed through a thermal diffusion method, laser doping, ionimplantation, or the like.

The second areas 122 may have a doping concentration ranging from 5E18to 1E20 and a depth ranging from 0.3 μm to 1 μm.

Next, (a) and (b) of FIG. 5 show side views of the solar cell 100. Asshown in (a) of FIG. 5, a first mask 200 including openings 210corresponding to positions, at which a plurality of front finger lines140 as described above are to be formed, is positioned on the substrate110, and the first impurity is secondarily doped through ionimplantation, or the like, to form the first areas 124 of the emitterlayer 120 as shown in (b) of FIG. 5.

The first areas 124 formed through the secondary doping are formed tohave a second doping concentration higher than that of the second areas122. The second doping concentration may range from 1E19 to 1E21. Inthis manner, contact resistance with the front finger lines 140 can bereduced (to be described). Accordingly, the selective emitter structurecan be obtained.

Also, the doping thickness of the first areas 124 may range from 0.5 μmto 2 μm in order to prevent the shunt with the front finger lines 140and to reduce or prevent an increase in the recombination of minoritycarriers.

(a) of FIG. 6 shows a plan view of the solar cell 100 and (b) of FIG. 6shows a side view of the solar cell 100. Then, as shown in FIG. 6, thefourth areas 152 (see (b) of FIG. 6) of the back surface field 150 asdescribed above are formed by using a second mask 300 covering edges 310(see (a) of FIG. 6) of the rear surface of the substrate 110. The secondmask 300 may be in a shape of a rectangular hoop.

The fourth areas 152 reduce or prevent the recombination of electron andhole pairs separated from the rear surface of the substrate 110. Thefourth areas 152 may be formed by doping the second impurity having thesame conductivity type as that of the substrate 110 through a thermaldiffusion method, laser doping, ion implantation, or the like. Thefourth areas 152 have a doping concentration ranging from 5E18 to 1E20and depth ranging from 0.3 μm to 1 μm.

Also, since the second mask 300 used to form the fourth areas 152 coversthe areas of the substrate 110 within the distance T from the edgeportion 310 on the rear surface of the substrate 110, a connectionbetween the emitter layer 120 on the side of the substrate 110 and thefourth areas 152 can be prevented. Thus, an additional edge isolationprocess for insulating the front and rear surfaces of the substrate 110can be omitted.

Accordingly, as described above with reference to FIG. 3, the backsurface field 150 is formed to be separated by the distance or gap Tfrom the edge of the rear surface of the substrate 110. In thisinstance, it is noted that the distance T between the back surface field150 and the edge of the rear surface of the substrate 110 ranges from 2μm to 300 μm. In this instance, (a) of FIG. 6 shows the section takenalong line A-A′ of the solar cell in FIG. 1.

Thereafter, as shown in (a) of FIG. 7, a third mask 400 includingopenings 410 corresponding to positions, at which the plurality of rearfinger lines 170 as described above are to be formed, is positioned onthe rear surface of the substrate 110 and a second impurity issecondarily doped through ion implantation, or the like, to form, asshown in (b) of FIG. 7, the third areas 154 of the back surface field150.

In this instance, the number of openings 410 formed on the third mask400 may be greater than the number of openings 210 formed on the firstmask 200.

The third areas 154 formed through the secondary doping are formed tohave a fourth doping concentration ranging from 1E19 to 1E21 higher thanthat of the fourth areas 152. Thus, contact resistance with the rearfinger lines 170 is reduced, improving the photoelectric conversionefficiency of the solar cell 100. Also, the back surface field 150 hasthe same structure as the foregoing emitter layer 120, so the solar cell100 according to an embodiment of the invention is a bifacial solar cellhaving the structure of the selective emitter layer 120 and theselective back surface field 150.

Meanwhile, a doping thickness of the third areas 154 may range from 1 μmto 2 μm in order to prevent a shunt with the rear finger lines 170 andto reduce or prevent an increase in recombination of minority carriers.

The emitter layer 120 and the back surface field 150 as described aboveare formed by performing the doping process twice, but the embodiment ofthe invention are not limited thereto, and the emitter layer 120 and theback surface field 150 may be formed through, for example, ionimplantation using a comb mask, or the like, at a time.

FIG. 8 shows a comb mask. With reference to FIG. 8, a combo mask 500includes a support portion 510 and a plurality of ribs 520 extendingfrom the support portion 510. Slots 530, i.e., openings, are formedbetween the ribs 520.

For example, a method for forming the emitter layer 120 by using thecomb mask 500 will now be described. The position of the comb mask 500is fixed on the substrate 110 and a first impurity is implanted into thesubstrate 110 through the slots 530 to form the first areas 124 of theemitter layer 120. After the formation of the first areas 124, thesubstrate 110 positioned under the comb mask 500 is moved and the firstimpurity is continuously implanted into the substrate 110 to form thesecond areas 122. In this instance, a doping concentration of each ofthe first areas 124 and the second areas 122 may be adjusted byregulating a period of time during which ions are implanted, the amountof doped ions, ion acceleration energy, and the like.

Next, as shown in FIG. 9, the first anti-reflective film 130 and thefront finger lines 140 are formed on the emitter layer 120, and thesecond anti-reflective film 160 and the rear finger lines 170 are formedon the back surface field 150. Also, a passivation layer may beadditionally formed on the emitter layer 120 and on the back surfacefield 150.

The first anti-reflective film 130 and the second anti-reflective film160 may be formed through vacuum deposition, chemical vapor deposition,spin coating, screen printing, or spray coating, but the embodiments ofthe invention are not meant to be limited thereto.

In order to form the front finger lines 140, paste for a front electrodemay be screen-printed on positions, at which the front finger lines 140are to be formed, by using a mask and then thermally treated to form thefront finger lines 140. In this instance, as silver included in theprinted paste is changed into a liquid phase at a high temperature andthen recrystallized into a solid phase through certain processes, theprinted paste is connected with the first areas 124 of the emitter layer120 according to a fire through phenomenon penetrating the firstanti-reflective film 130.

Also, the first anti-reflective film 130 and/or the passivation layeris/are irradiated with a laser beam through laser ablation so as to beremoved, and a seed layer is deposited with nickel (Ni), or the like.Then, the front finger lines 140 may be deposited through plating,sputtering, or the like, so as to be formed. Besides, the front fingerlines may be also formed through laser firing, screen printing afterlaser ablation, or the like, but the embodiments of the invention arenot meant to be limited thereto.

The rear finger lines 170 may be formed in the same manner as that ofthe front finger lines 140, so a detailed description thereof will beomitted.

Meanwhile, in order to reduce or prevent a reduction in resistance ofthe solar cell 100 and a degradation of the photoelectric conversionefficiency of the solar cell 100, the number of the rear finger lines170 may be different from that of the front finger lines 140. Forexample, a larger number of rear finger lines 170 than that of the frontfinger lines 140 may be formed.

Also, the ratio of the distance between two adjacent rear finger lines170 to the distance between two adjacent front finger lines 140 may begreater than 1 and smaller than 5. If the ratio of the distance betweentwo adjacent rear finger lines 170 to the distance between two adjacentfront finger lines 140 is 5 or greater, the overall area of the rearfinger lines 170 would be increased to increase the recombination ofelectron and hole pairs due to a metal, to potentially reducing the opencircuit voltage Voc of the solar cell 100. Conversely, if the ratio ofthe distance between two adjacent rear finger lines 170 to the distancebetween two adjacent front finger lines 140 is 1 or smaller, a movementpath of carriers is increased to potentially reduce the efficiency ofthe solar cell 100.

In this instance, the distance between two adjacent rear finger lines170 may range from 0.5 μm to 2.5 μm in order to reduce or prevent therecombination of electron and hole pairs and to reduce or prevent anincrease in resistance.

FIG. 10 is a sectional view of a solar cell according to an embodimentof the invention, and FIG. 11 is an enlarged view of a portion ‘C’ ofFIG. 10. With reference to FIG. 10, a solar cell module 500 according toan embodiment of the invention may include a plurality of solar cells550, a plurality of ribbons 543 electrically connecting the plurality ofsolar cells 550, a first sealing film 531 and a second sealing film 532hermitically sealing the plurality of solar cells 550 at both (oropposite) sides, a front substrate 510 protecting one surface of each ofthe solar cells 550 and a rear substrate 520 protecting a rear surfaceof each of the solar cells 550.

The solar cells 550 are the same as described above with reference toFIGS. 1 to 3, so a detailed description thereof will be omitted. Theplurality of solar cells 550 are electrically connected to each other bythe ribbons 543, forming a string 540. For example, two lines of theribbons 543 may be attached to upper and lower portions of the solarcells 550 through a tabbing process to electrically connect theplurality of solar cells 550. Namely, in performing the tabbing process,flux is applied to one surface of each of the solar cells 550, theribbon 543 is positioned on each of the flux-applied solar cells 550,and then firing process is performed.

Or, as shown in FIG. 11, a conductive film 560 may be attached betweenone surface of each of the solar cells 550 and each of the ribbons 543,and be thermally compressed to connect the plurality of solar cells 550in series or in parallel. The conductive film 560 may include a basefilm 562 and conductive particles 564 dispersed in the base film 562.The conductive particles 564 are exposed from the base film 562according to (or when subjected to) the thermo-compression, and thesolar cells 550 and the ribbons 543 may be electrically connected by theexposed conductive particles 564.

The base film 562 may be made of a thermosetting resin having excellentbondability and insulating characteristics, e.g., an epoxy resin, anacryl resin, a polyimide resin, a polycarbonate resin, or the like, andthe conductive particles 564 may be gold, silver, nickel, copperparticles, or the like, having excellent conductivity. In particular,the conductive particles 564 may be particles obtained by plating theforegoing metal particles on a polymer material, or the like.

When the plurality of solar cells 550 are connected by the conductivefilm so as to be modularized, a processing temperature can be lowered toprevent the string 540 from being bent.

With reference back to FIG. 10, the first and second sealing films 531and 532 hermitically seal the plurality of solar cells 550 at both (oropposite) sides thereof. The first sealing film 531 and the secondsealing film 532 may be bonded by lamination to prevent penetration ofmoisture or oxygen which may negatively affect the solar cells 550.

Also, the first sealing film 531 and the second sealing film 532 mayallow each element of the solar cells 550 to be chemically coupled. Thefirst sealing film 531 and the second sealing film 532 may be made of anethylene vinyl acetate copolymer (EVA) resin, polyvinyl butyral,ethylene vinyl acetate partial oxide, a silicon resin, an ester-basedresin, an olefin-based resin, or the like.

The front substrate 510 is positioned on the first sealing film 531 toallow solar light to be transmitted therethrough, and it may be temperedglass in order to protect the solar cells 550 against an externalimpact, or the like. More preferably, the front substrate 510 may be alow iron tempered glass containing a smaller amount of iron in order toreduce or prevent a reflection of solar light and increase transmittanceof solar light.

The rear substrate 520, which is to protect the solar cells 550 at therear side of the solar cells 550, performs functions such aswaterproofing, insulating, and filtering of ultraviolet rays. The rearsubstrate 520 may be a TPT (Tedlar/PET/Tedlar) type rear substrate, butthe embodiments of the invention are not meant to be limited thereto.The rear substrate 520 may be made of a transparent material allowingsolar light to be made incident thereto.

In various embodiments of the invention, a width of the plurality offront finger lines 140 may be the same or different from a width of theplurality of rear finger lines 170. Also, the number of the plurality ofrear finger lines 170 positioned on the entire rear surface of thesubstrate 110 is different from the number of the plurality of frontfinger lines 140 positioned on the entire front surface of the substrate110. Additionally, one or more of the third areas 154 of the backsurface field 150 may be wider than one or more of the first areas 124of the emitter layer 120.

As the invention may be embodied in several forms without departing fromthe characteristics thereof, it should also be understood that theabove-described embodiments are not limited by any of the details of theforegoing description, unless otherwise specified, but rather should beconstrued broadly within its scope as defined in the appended claims,and therefore all changes and modifications that fall within the metesand bounds of the claims, or equivalents of such metes and bounds aretherefore intended to be embraced by the appended claims.

What is claimed is:
 1. A method for fabricating a solar cell, the methodcomprising: forming an emitter layer by doping a first impurity having asecond conductivity type, opposite a first conductivity type, on a frontsurface of a substrate having the first conductivity type; forming aback surface field by doping a second impurity having the firstconductivity type on a rear surface of the substrate; and forming aplurality of front finger lines in contact with the emitter layer and aplurality of rear finger lines in contact with the back surface field,wherein the emitter layer has a selective emitter structure, the backsurface field has a selective back surface field structure, and thenumber of the plurality of rear finger lines positioned on the rearsurface of the substrate is different from the number of the pluralityof front finger lines positioned on a front surface of the substrate. 2.The method of claim 1, wherein the number of the plurality of rearfinger lines is greater than the number of the plurality of front fingerlines.
 3. The method of claim 2, wherein a ratio of a distance betweentwo adjacent rear finger lines among the plurality of rear finger linesto a distance between two adjacent front finger lines among theplurality of front finger lines is greater than 1 and smaller than
 5. 4.The method of claim 1, wherein the emitter layer and the back surfacefield are formed through ion implantation.
 5. The method of claim 1,wherein the forming of the emitter layer comprises: primarily doping thefirst impurity with a first doping concentration on the entirety of thefront surface of the substrate; positioning a first mask includingopenings corresponding to positions at which the plurality of frontfinger lines are to be formed, on the substrate; and secondarily dopingthe first impurity with a second doping concentration higher than thefirst doping concentration at the positions the plurality of frontfinger lines are to be formed.
 6. The method of claim 1, wherein theforming of the back surface field comprises: positioning a second maskcovering an edge portion of the rear surface of the substrate on therear surface of the substrate and primarily doping the second impuritywith a third doping concentration on the rear surface of the substrate;and positioning a third mask including openings corresponding topositions at which the plurality of rear finger lines are to be formed,on the rear surface of the substrate, and secondarily doping the secondimpurity with a fourth doping concentration higher than the third dopingconcentration at the positions the plurality of rear finger lines are tobe formed.
 7. The method of claim 1, wherein the back surface field isseparated from an edge of the rear surface of the substrate by a gap of2 μm to 300 μm.
 8. The method of claim 1, further comprising: forming ananti-reflective film on the emitter layer and an insulating layer on theback surface field.